I’ve recently moved in to a new apartment that sorely lacks furnishing. Instead of going on a shopping spree at IKEA, I’m using the CNC tools at Autodesk Pier 9 to furnish my new apartment. I don’t have any background in furniture design and limited CNC experience, so I’m also pursuing this project as a way of learning more about furniture and the CNC tools at the Pier 9 shop.
To get started, I chose the simplest piece of furniture: the stool. For the unfurnished apartment, the stool is a flexible and useful item - obviously, you can sit on a stool - but it can also serve as a makeshift bedside and dining table.
In this Instructable, I discuss the design of the stool, water jet cutting 0.75” birch plywood, sanding, staining and assembly.
A few quick shoutouts: Che-Wei and Taylor inspired the single-fastener design. They designed a flatpack, single-fastener stool in How To Make at MIT. Josh Myers was generally awesome and super helpful and showed me how to use the vacuum table and the router table. Martin Horn provided some great advice on how to cut plywood using the water jet and how to stain the plywood. Thanks everyone!
Step 1: Materials, Fabrication, and Assembly
Before designing the stool, I considered a few aspects of the design:
- Materials - What material is inexpensive, strong and machinable?
- Fabrication - How would I build the parts?
- Assembly - How would the parts come together to form the stool?
I decided to use 3/4” 5-ply birch, a strong, relatively machinable, inexpensive ($50-60 for a 4’ x 8’ sheet) and readily available plywood. It’s often used for CNC furniture projects.
In this project, I decided to cut the birch ply using a water jet CNC machine. The water jet uses a high-pressure stream of water and garnet to abrasively cut through nearly any material. The water jet cuts 0.75” birch ply nearly as quickly as a large laser cutter (such as the 400 Watt Metabeam) but without the burn marks. Compared to a Shopbot CNC router, the water jet is much faster and simpler to operate and can cut corners with smaller radii. However, the water jet has two major drawbacks for this project: the waterjet only allows for profile cuts (cutting all the way through the material). Therefore, no countersunk holes or 3D contours are possible. Secondly, the water jet cuts through materials using garnet and high pressure water - both of which could stain the wood.
Finally, I wanted to make the stools “flat-pack," allowing the stools to be easily shipped, assembled and de-assembled. This eliminated glue and screws as fastener options. Instead, I researched alternative assembly options and ended up borrowing a fastening technique from my close friends, Che-Wei and Taylor. They designed a stool that features a single long bolt that presses the legs into the top seat. Check out their design here.
Step 2: Stool Design in Autodesk Inventor
I designed the stool based on the classic Alvar Aalto design. Aalto designed the simple, three legged, stackable stool by fastening bent plywood legs onto a round top. I like the design for its simplicity and timelessness.
I used Autodesk Inventor to design the stool. Inventor is a parametric solid modeling tool that uses a 2D Sketch -> 3D Part -> Assembly workflow.
To design a flat-pack plywood stool fastened using a single bolt, it was clear that I would need to use press (or interference) fits for connecting the parts. Sizing the interference joints depends on the material thickness, which can vary slightly in each piece of plywood. Therefore, it is critical to update to geometry of the CAD model based on the thickness of the actual plywood being used for each stool.
One way to do this is to set up a parameter for the material thickness. Inventor uses an Excel spreadsheet to create variables that can be linked to dimensions. I created parameters for the material thickness, stool height, and width. These variables were used throughout the parts to inform the dimensions of each sketch.
The final design can be seen in the images below. The Inventor and Excel files have been uploaded and I encourage you to download and use them.
A few additional features of the design:
I doubled the thickness of the plywood throughout the stool for added strength. I like the aesthetic of a thicker, sturdier-looking stool. I also had to use two layers of plywood for the bolt assembly. In Che-wei and Taylor’s design they countersunk the bolt into the seat using a Shopbot CNC. A water jet can’t make a countersunk hole in plywood, so I designed the top layer of the seat with a press fitting to the head of the hex bolt and the bottom layer with a press fitting to the shaft diameter.
For the press fit, I used the same dimensions for the female and male part. For example, to fit a 3” square peg into a 3” square hole I designed both with exactly 3” dimensions. The wood is somewhat pliable and a square peg with the same dimensions as a square hole can snugly fit inside. The OMAX software adjusts for the kerf of the water jet, meaning a 3” square in CAD should result in an actual 3” square.
I used a 3/8” x 3.5” hex bolt (something like this). I used this bolt because it was long enough for the design and it happened to be laying around the Pier9 shop.
To prep the parts for cutting, I exported the necessary faces in DXF.
All the inventor files can be found in the attached zip file.
Step 3: Prepping to Cut the Stool in Omax Layout
I imported each DXF into Omax Layout and prepped the tool paths (.ord files) for water jet cutting. I set the quality equal to 4 (5 is highest quality. Higher quality = slower cuts). In retrospect, it would have been smart to do a small test cut with each quality. Each DXF is fairly straightforward so prepping the files was as simple as (1) setting quality, (2) auto-pathing, and (3) setting the start point.
Step 4: Cutting Out the Stool on the Waterjet
I then prepared the Pier 9 OMAX water jet for the cut with the standard set of activities: (1) activated the charge pump, (2) pressurized the garnet hopper, (3) loaded the plywood into the water jet, (4) fastened in into place onto the bed, (5) homed the xy and z coordinates, (6) loaded the .ord file created by OMAX layout, (7) set the material and material thickness in OMAX Make, and (8) checked the extents of the tool path to make sure the water jet head would not run into any fasteners.
The one non-standard thing I did while operating the water jet was keeping the water level lowered. In normal operation of the water jet, the water level is raised above the material to reduce splatter and sound. I was afraid that soaking the plywood in the raised waterbed would discolor and warp the plywood. Instead, I cut the plywood with the water level lowered. Despite this precaution, the water jet kicked up water, garnet, and “water jet sludge” onto the material surface, leading to some discoloration.
After the cut was completed, I moved the water jet head away from the plywood and removed the pieces. Some of the surfaces of the cuts were rather jagged and the cut was obviously drafted by a couple degrees (typical of a water jet). The surface of the plywood was fairly discolored from the water jet process.
Step 5: Drying the Stool Pieces Overnight
I dried the pieces overnight on my desk. The pieces dried for about 16 hours total.
Step 6: Sanding the Stool Legs and Seat to Remove Discoloration and Jagged Edges
Before gluing the pieces together, I sanded each piece to remove as much of the discoloration from the surface as possible and to smooth the jaggedness of the water jet cut.
I first sanded the surfaces of the plywood using a handheld rotary sander. I used the vacuum bed in the Pier 9 wood working workshop.
I sanded each surface first with a low-grit sandpaper until nearly all of the discoloration was removed (4-6 minutes per side per piece) and then followed up with a high grit sand paper. Overall, sanding the surfaces took approximately 45 minutes for the 9 pieces.
Next, I sanded the edges of each piece to remove the jaggedness from the water jet cut. For the legs, I clamped each leg down using the wooden vice attached to the wooden workbench and then hand sanded each leg with a low-grit paper followed by a high-grit paper. Each side required about 1-2 minutes of sanding.
I then used a belt sander to sand the curved edges of the legs and seat.
Finally, to give the top surface of the seat a smooth finish, I gave the top piece of the seat a quick hand sanding using a high-grit sand paper (400-600 grit).
Step 7: Gluing Together the Legs and Seat
To glue the pieces together, I used a Titebond wood glue and numerous clamps. The glue has an approximately 5 minute working time and I used 3-4 clamps per glued piece. I let the glue set for 20 minutes before moving onto staining.
Step 8: Staining Using Linseed Oil
Martin from the shop recommended using Boiled Linseed Oil to stain the plywood.
I used microfiber and shop rags to apply the oil. I let the oil dry for about a hour before assembly.
Step 9: Assembly
Now for the easy part: Assembly!
I ended up using a small mallet to ensure the top of legs were flush again the seat.
Step 10: Making Two More Stools to Complete the Set!
I was pretty happy with the results of the first stool, so I decided to make two more stools to round out the set.
For these two stools, I performed the exact same steps, except for one change. I decided to fillet the top and bottom edges of the seat.
I filleted the top and bottom edges of the seat using the router table. I picked a router bit with a fillet radius that was roughly half the material thickness (3/8” radius). For both the top and bottom seat pieces, I filleted the side of the wood that was face-down on the water jet. Why? The edge of the wood that was face down during the water jet cut was rather jagged, while the edge that was face up was smooth. The smooth edge should ride against the reference surface - the bearing of the bit - in order to have a smooth fillet cut. If the piece were flipped, the jagged surface of the edge would ride against the bearing, causing a jagged, uneven fillet cut.
After filleting the surfaces I proceeded to sand, glue and stain the stools. I hand sanded the filleted surfaces of the top seat prior to staining.
Step 11: Final Set of Stacked, CNC-fabricated Aalto Stools
This project was a great introduction to 2D CNC at Pier 9. Stay tuned for more furniture projects that use some of the other Pier 9 tools, including the Shopbot CNC Router and the 5-axis DMS Router.